'Abdullah b. 'Umar reported Allah's Messenger (may peace be upon him) as saying:
"The example of a man who has memorised the Qur'an is like that of a hobbled camel. If he remained vigilant, he would be able to retain it (with him), and if he loosened the hobbled camel it would escape".
~Sahih Muslim-Book 4, Number 1722 ~
Meaning, the Quraan is lost from the memory very quick if it is not taken care of by keeping it on our tongues and in our heart, reviewing and reciting it everyday some part of it in every prayer so to never let it go. Just like an animal escapes when you let it to be on its own, and then imagine the effort u would have to make to get that lost animal back.
Similarly, Quraan once escaped from the memory and mind and heart, it is very stressful to bring it back to ur mind like it was before. (I am experiencing this too, i realize its a lot to handle for me now, now that i have many chapters in my memory Alhamdulilah, thats why i am here writing all this for myself).
And ofcourse this is not just with the memory of the words of the quraan, its understanding would also be lost if we dont stick to it and dont impart to others what we understood of it.
Why would anyone knowing what a treasure it is, ever let it go?
Here is the test. When my heart is attached to the Quraan, and I read it regularly and feel at peace, as compared to when i recite it only when i m to memorize, that makes the difference. Allah SWT could have made me have it memorized it once and never ever forget it, but I m to stick to it, keep it tied tight with my own struggle, have its blessings all the time by reciting its words. Getting a new meaning that relates to me myself everytime i read the verses, finding more wisdom in them ... and so much more, which i would miss otherwise.
May Allah help retain Quraan in our memories and hearts, as much as we know of it, and increase us in its knowledge and make it a witness in our favour on the Last Day, Aameen!
"And We have indeed made the Qur’ân easy to understand and remember"